1. Field of the Invention
This invention relates to thermosetting resin compositions that are useful as high performance and high layer count, multilayer printed circuit board (PCB), prepregs, resin coated copper (RCC), film adhesives, high frequency radomes, radio frequency (RF) laminates and various other composites made from resin compositions.
2. Description of the Prior Art
Advances in electronic devices continue to approach the limit of printed circuit board (PCB) technologies. The performance requirements for composite and laminate materials are becoming more stringent. In the “cloud computing”, Smartphone industries and wireless communication (4G and 4G LET Advanced) for example, high speed high frequency circuits require substrates with difficult to attain electrical properties, such as ultra low dielectric loss and low dielectric constant. Current composite materials fail to meet some of the most critical requirements such as dielectric loss in high-speed communications. As frequency increases, the amount of signal loss to the substrate becomes more significant. As a result, materials are required that will give PCBs the required electrical properties for rapid transmission of high frequency signals with signals integrity, while maintaining the thermal, physical and mechanical properties desirable for PCBs.
Various composite materials employed include Polytetrafluoroethyene (PTFE). Although PTFE is an excellent low dielectric material, it suffers from significant processing problems such as a lack of fluidity due to high melt temperature and viscosity; the inability to form multilayer boards (high layer count) and low glass transition temperature (Tg). These processing problems limit PTFE use in radio frequency (RF) applications and limited multilayer digital substrate applications. PTFE is also a high price material, which further prohibits its use in the mass production of consumer electronics.
Epoxy resins on other hand are the industry standard for PCB manufacture due to low cost and good processing conditions. However poor electric properties and low glass transition (Tg) limit epoxy to use as high speed communication and high temperature integrated circuit (IC) substrates suitable for tablets and other computing industries. The most common mass production epoxy laminates are FR-4, FR-5 and various enhanced epoxies.
Another composite material employed includes Cyanate esters (CE) which are known for generating thermoset materials with mid range dielectric constants and dielectric loss properties. CEs are considered useful for high performance substrate applications. Benefits of these materials include process-ability similar to epoxy laminates (FR-4) and good thermal properties in dry conditions.
However, cyanate resins have been disadvantageous in several aspects. Typical prior art cyanate resins include:
Blending CEs with thermoplastics and elastomers to form resins. Disadvantageously, this blend has been found to generate semi-interpenetrating networks rather than uniform resins and often causes phase separation between cyanate and modifier domains. It has been found that the use of hydroxylated polybutadiene (HPBD) in modifying CEs by mixing/blending the two materials and directly curing them generates a material with significant phase separation, which results in poor thermal resistance. It has further been determined that liquid HPBD are incompatible with many CEs and that the use of a copolymer to combine and blend the materials is required.
Various elastomer modified cyanate esters exhibit low Tg and high tackiness and are unsuitable for multilayer boards. For example:
U.S. Pat. No. 4,780,507 to Gaku et al, discloses a thermosetting resin composition comprising a thermosetting cyanate ester resin composition (A) and a butadiene based copolymer (B)(i) or an epoxy resin-modified butadiene based copolymer (B)(ii), in which component (B)(i) or (B)(ii) are used for modifying component (A). Components are first pre-polymerized into a resin and then form a non-tacky resinous material by controlling the time and temperature of the reaction between the different components. The use of solid polybutadiene-co-vinylaromatic polymers to modify CEs is disclosed. However, these materials possess glass transition temperatures below 215° C.
U.S. Pat. App. Pub. 2013/0245161 A1 disclsoes resins incorporating epoxies, cyanate esters and polybutadiene-styrene-divinylbenzene terpolymers, however these suffer from extremely low Tgs between 100 and 135° C. and high dissipation factors >0.005.
CN101824157 to Baixing et al. discloses a method for modifying cyanate ester resin by hydroxyl-terminated polybutadiene, which comprises the following steps: adding hydroxyl-terminated polybutadiene rubber into the cyanate ester resin with the addition quantity of 5 to 30 weight percent; heating and melting the mixture to be uniformly mixed; heating the mixture to 120+/−15 DEG C. for pre-polymerization for 10 to 60 min; carrying out casting and curing; uniformly heating at 130 to 200 DEG C. for 7 to 10 h for curing; and obtaining the modified cyanate ester resin. However, these CEs have been found to have low TG and high tackiness and thus lack usefulness in multilayer PCB technologies.
A significant drawback of CE resins is that they suffer from moisture uptake. Moisture plays a significant role in the failure of high frequency applications. Water contributes high Dk (80) and has high polarity; as a result even a small amount of water can have a detrimental effect on the physical and electrical properties of substrate materials. If water reacts with the resin system it may contribute to delamination during thermal performance. The resins of the invention have reduced moisture uptake due to the reaction with modifying resins. Furthermore, the commercial CEs are brittle due to a tight network structure and as a result thin substrates made from CE are fragile for drop tests in smart phone and other portable devices.
For examples, see:
U.S. Pat. No. 8,404,764 to Yu et al. discloses a resin composition comprising (A) 100 parts by weight of cyanate ester resin; (B) 5 to 25 parts by weight of nitrogen and oxygen containing heterocyclic compound; (C) 5 to 75 parts by weight of polyphenylene oxide resin; and (D) 5 to 100 parts by weight of oligomer of phenylmethane maleimide. By using specific components at specific proportions, the resin composition is taught to offer the features of low dielectric constant and low dissipation factor and can be made into prepreg that may be used in printed circuit board. Reports therein have been provided showing a polyphenylene oxide/cyanate ester resin that exhibits dissipation factors near 0.0055, and which display Tgs below 185° C.
U.S. Pat. App. No. 20070203308A1 to Mori et al. and U.S. Pat. No. 6,245,841 to Yeager et al. discloses curable compositions used in circuit boards, structural composite, encapsulating resins, and the like, comprise at least one compound selected from the group consisting of cyanate esters and cyanate ester prepolymers, a flame retardant which is substantially toluene soluble and substantially free of hydroxy residues in the cured state, and a curing catalyst. Though issues of moisture properties were addressed, the processing of this product is difficult using conventional procedures due to high melt viscosity. Furthermore, the thermoplastic (polyphenylene oxide, allyl, or liquid crystal polymers (LCD)) is only soluble in exotic solvents (toluene, xylene, etc.).
U.S. Pat. No. 5,571,609 to Lawrence St. et al. discloses an electrical substrate material comprising a thermosetting matrix which includes a polybutadiene or polyisoprene resin and an unsaturated butadiene or isoprene containing polymer in an amount of 25 to 50 vol. %; a woven glass fabric in an amount of 10 to 40 vol. %; a particulate, preferably ceramic filler in an amount of from 5 to 60 vol. %; a flame retardant and a peroxide cure initiator. A preferred composition has 18% woven glass, 41% particulate filler and 30% thermosetting matrix. The foregoing component ratios and particularly the relatively high range of particulate filler is an important feature of this invention in that this filled composite material leads to a prepreg which has very little tackiness and can therefore be easily handled by operators. Disadvantageously, this product is not suitable for high layer count boards due to the lack of adhesion of the hydrocarbon.
U.S. Pat. No. 7,425,371 discloses a thermosetting resin system that appointed to be useful in the manufacture of high performance prepreg, laminate and composite materials as well as the prepregs, laminates and composites made from the thermosetting resin composition. The reference discloses modifications of CEs with SMA but this composition lacks the electrical performance for 4G and beyond applications, and no example is given to illustrate any benefit derived by modifying CE with SMA. The large part of electrical properties was achieved by blending fused silica with epoxy, ester, elastomers such as SMA, and a flame retardant. Furthermore, the compositions claimed a chemically blended product, not a reaction intermediate.
Accordingly, there remains a need in the art for ultra low loss dielectric thermosetting resin compositions for high performance laminates. Particularly needed in the art are ultra low loss dielectric thermosetting resin compositions for use in high performance and high layer count, multilayer printed circuit board (PCB), prepregs, resin coated copper (RCC), film adhesives, high frequency radomes, radio frequency (RF) laminates and various other composites made from resin compositions. Further needed in the art is a thermosetting resin composition that exhibits excellent dielectric properties suitable but not limited for 4G and 3rd Generation Partnership Project (3GPP).